The present invention relates generally to motors and the vibrations or resonances which are set up within such motors; particular applications or examples will be given with respect to spindle motors used in disc drive assemblies, but the application of this invention is not limited to these specific examples. The invention discloses actuators for eliminating such resonances.
Most motors, indeed most rotating systems that are spinning about a fixed axis, have vibrations or harmonics which are set up and become part of the system, disturbing the overall stability and smooth operation of the system. Such problems are particularly acute in the disc drive industry, where a spindle motor mounts and supports a disc or disc pack for high speed rotation. The disk drive industry is continually seeking to obtain a head disc assembly (HDA) capable of operating with an increased track density which requires greater resistancy to shock and vibration.
As the operating demands on the HDA increase, problems associated with conventional HDA systems become performance limiting factors: for example non-repetitive run-out (NRR) associated with conventional ball bearings limits track spacing and, thus, reduces the track density at which the HDA can reliably operate. NRR is associated with the highly complex dynamic behavior of the hard disk drives: mechanical modes of the motor and the disc pack correspond to predicted mechanical resonance, which are in turn exited by ball bearing vibration. To reduce NRR magnitude, the vibrational characteristics of the drive have to be modified. Some standard solutions: to use a non-contact bearing (like magnetic or hydrodynamic bearings) which does not create any vibration and thus does not excite the resonance modes.
In the prior art, a number of efforts have been made to electronically damp vibration associated with a motor or with a transducer in a disc drive. The prior art to damp vibrations in a moving transducer in order to more quickly center it on a track includes U.S. patents to Song, U.S. Pat. No. 4,414,497; Sidman, U.S. Pat. No. 5,459,383; and Ravizza, U.S. Pat. No. 4,080,636. Each of these comprise elaborate circuitry for adding feedback loops to more quickly damp out the vibrations or movements in a moving transducer. All of these are not associated with problems of damping out vibrations in a motor or the disc itself, and also add considerable complexity and costs to the system.
Other patents have added mechanical or electromechanical elements to the motor itself in an effort to damp out vibrations in the motor. These patents include Hasigawa, U.S. Pat. No. 5,317,466; Bartec, U.S. Pat. No. 4,198,863; Clancey, U.S. Pat. No. 4,286,202. These patents are especially directed to the addition of mechanical or electromechanical elements to motors to detect and damp out vibration. Again, these prior art approaches have not proven to be effective in detecting the resonance modes which can exist in rotating motors and particularly disc drive spindle motors, and damping out such resonances. Further, they add considerable cost and complexity to the motor design.
Therefore, an object of the present invention is to damp out the resonances which occur in a rotating system such as a disc drive spindle motor.
A related objective is to stabilize the spin axis of a rotating system in a given position.
It is a further objective of this invention to accurately simulate the undamped resonant movements of the rotating system, and then to apply an out of phase force to the rotating system to damp out the resonant movements.
It is a further related objective of the invention to apply an out of phase force which accurately and repeatably damps or attenuates the resonant movements and thereby stabilizes the system.
In this invention, the term xe2x80x9cattenuationxe2x80x9d is directly related to the fact that the force applied is out of phase with the resonant movements within the system. This is as opposed to the idea of adding a force which directly opposes the resonances which are occurring within the system, which would thereby add stiffness to the system, and constitutes the approach taken by the prior art. This would be the approach taken by an electromagnetic bearing, or the like, being added to the system.
It is a further objective of this invention to demonstrate that such undampened movements may be sensed and then appropriate forces applied through circuitry to damp out these movements.
Another objective of this invention is to demonstrate that the damping actuator may comprise windings of a motor supporting the rotating system.
Another objective of the invention is to establish that the actuator for damping out movements may comprise specific windings of the motor supporting the rotating system having currents of selected magnitude and phase applied thereto.
Yet another objective of the invention is to demonstrate an approach for selecting such windings in a motor, and the direction and magnitude of current flow in the winding to provide an effective damping actuator.
Thus, in the present invention, resonant movements are first simulated, and then a derivative of that representation is utilized to define a damping signal, lagging in phase, which controls the application of force to the system to damp out the resonance within the system. Thus, the damping method taught by this invention comprises measuring the movements of the system in time, and then lagging that very same force by ninety degrees and applying that damping force to correct the tendency for that movement to occur. Thus, according to the invention a movement at a given velocity is countered by a counter movement at a given velocity, so that movements at high frequency are successfully damped out. The success of this approach is based at least in part on the fact that the derivative of the representation of the movement always has velocity as a factor in its representation of the resonant movements of the rotating system. Thus, at higher frequency movements, this damping approach is substantially more effective than the addition of stiffness to the system would be.
In the following detailed description, equations will be generated to demonstrate that a radial force to dampen resonant movements in a highly responsive manner can be generated using additional windings within a spindle motor which is used herein as an exemplary rotating system. The coils are grouped in phases and energized to produce a radial force as required. An appropriate exemplary circuit for taking the representation of the undesired movement and creating and damping force through the application of current to the additional windings is also disclosed.
This detailed description is for example only; the principles of the analysis and invention could be applied utilizing the windings already present in the motor. For example, the windings could be tapped, and currents necessary to generate the stabilizing radial force be added to the normal motor driving currents. Alternatively, the regular driving currents could be turned off for a very brief period, and the calculated currents to create a radial force imposed on the same windings; the two currents could be alternated rapidly so that the momentum of the motor is maintained by these driving currents while the radial force is created by the calculated currents.